1. Field of the Invention
The present invention relates to handoffs and dropoffs in a cellular communication system. More particularly, the present invention relates to establishing whether a cellular radiotelephone with a weak signal should be handed off or dropped off.
2. Description of Related Art
A radiotelephone system consists of many base stations, also known as cell sites, each coupled to a mobile telephone switching office (MTSO) connected to the public switched telephone network (PSTN). Each base station is equipped with an antenna that receives and radiates radiotelephone signals in order to communicate with cellular radiotelephones and switch the signals from the radiotelephones to the mobile telephone switching office. The base station also provides the proper channels for the radiotelephones, thus enabling communication with the base station. The mobile telephone switching office switches the signals from the base station to the PSTN and vice versa.
Code Division Multiple Access (CDMA), and spread spectrum systems in general, were previously dismissed as unworkable in the mobile radio environment because of the near-far problem. Near-far problem occurs in cellular communication systems where different transmitters interfere with the base station transmitter. In the mobile radiotelephone environment, some users may be located near the base station, and others may be located far away. The propagation path loss difference between those extreme users can be many tens of dB. This reduces overall, average link capacity. The anti-jamming capacity of spread spectrum systems, such as CDMA systems, aids in such situations, but unfortunately not enough to overcome the disparity between a strong nearby interfering signal and an attenuated remote signal. CDMA systems use codes to separate one signal from another. If there are enough different codes available, the limit on the number of users is determined by a system jamming margin, because a receiver will work as long as the composite power of all unwanted signals does not exceed the power of the desired signal by more than a jamming margin.
It had been assumed that all mobile radiotelephones in a cell had to transmit constant power. If, however, rather than using constant power the transmitters can be controlled in such a way that their power level can be changed so that the received power levels from all the users are roughly equal, the benefits of the spread spectrum system are realized. In systems where the received power is controlled, the subscribers can occupy the same spectrum, and the hoped-for benefits of interference averaging accrue. However, the systems that employ power control for intracell solution of the near-far problem suffer from significant loss in receiver sensitivity because all signals must arrive at the receiver (base station) at equal power levels, to maximize the number of simultaneous users. Power control may be accomplished in each radiotelephone by verifying its distance from the base station and adjusting its output power accordingly, so that the base station receives the signal at the predetermined signal power level. Otherwise, the weakest mobile transmitter, especially near the edge of a cell, dictates performance of the other cell users, which than have to change their transmitting power to accommodate the weak user.
In order to prevent increase of transmitting signal power of a radiotelephone at the edge of a cell in order to improve transmission and thus cause jamming of the base station by requesting all units in the new cell to increase their transmitting power to that level, handoff to another cell base station is performed by the current cell base station. However, if a user's signal is very weak, the radiotelephone has to be dropped off from the cell by the base station before it wipes out the entire network. Very weak users are not handed over to another cell base station because their performance will be poor near the edge of the adjacent cell after the handoff.
In CDMA systems with power control, if adjacent cells have a base station that can be heard by several base stations, a radiotelephone in that cell has to be in communication with all those base stations in order to accomplish a soft handoff. Soft handoff, also called make-before-break, is performed in CDMA systems where all cells use the same frequency (as defined by the IS-95A standard), and it is possible to make connection to the adjacent cell before leaving the current cell. Soft handoffs require less power, which reduces interference and increases capacity. In a soft handoff, control is handed over to the most convenient base station to service the user, while all other base stations stay in full communication with the user. When the radiotelephone reaches the edge of the cell, the controlling base station assists in its handoff to transfer control to another cell. The base station, however, does not know the location of the radiotelephone and it has to page all radiotelephones in the cell and wait for the arrival of their response, which takes time, and determine the candidate for dropoff or handoff from the response.
In a CDMA system with power control, each participating cell in a soft handoff transmits the same traffic stream to the mobile radiotelephone, bit-for-bit. They do so on any available code channel. Each base station chooses a code channel simply on the basis of availability. The radiotelephone must implement, in its rake receiver, multiple fingers that are capable of "tuning" to any of the, typically 63, available code channels. Embedded in the forward CDMA channel are the reverse power control bits. Each power control bit is interpreted as a command to raise or lower power by an increment of approximately 3/4 dB. Each base station makes power control decisions independently. The mobile station is responsible for demodulating the power control bits and raising or lowering its power accordingly. The goal of the power control is to maintain the reverse link transmit power at the lowest possible level commensurate with adequate error performance. The mobile radiotelephone is thus required to interpret the power control bits, which will often disagree, as requiring an increase in power only if all base stations in the handoff say "up". If any participating base station says "down", then the mobile radiotelephone is required to reduce power. This rule is sometimes called "OR of the downs".
CDMA systems uses Mobile Assisted Handoff (MAHO). In practice, this means that the mobile radiotelephone continuously searches for a pilot code using a PN correlator specifically designated for this purpose. All base stations use the same code. If the mobile radiotelephone already has a notion of CDMA system time, as it does if it is already involved in a call, then it can report the relative timing of a newly detected pilot. What distinguishes base stations from one another is the phase of their pilots. The period of each pilot is 26.667 ms. They are separated by a minimum of 64 chips, which is about 52 ms or about 15 km at the speed of light. The mobile radiotelephone timing will normally be sufficiently good that a reported pilot offset unambiguously identifies the base station it has detected.
The mobile radiotelephone reports pilots on the basis of their pilot-to-interference ratio (PIR). The PIR is compared to an absolute threshold to determine when it should be reported as a handoff candidate. This absolute or first threshold is a parameter that the mobile radiotelephone obtains from the overhead messages broadcast by the base stations. When a pilot crosses the first threshold, its presence is reported, via a message, to the network. The network adds that base station to the so-called active set, which is the set of base stations that are participants in the soft handoff of the mobile radiotelephone in question. A second threshold which is not absolute but relative is compared to the difference between the largest PIR in the active set and the PIRs of all other members. When any of them falls below this threshold, another message is transmitted. The normal result is that the base station in question will be dropped from the active set, and that will be reported to the mobile radiotelephone by a signaling message.
The effect of the two thresholds, one absolute, the other relative, is to ensure that any station that is able to contribute in any significant way to the overall signal-to-noise ratio is in the active set with high probability. Conversely, a base station is dropped only when it has deteriorated far below the best station. If the best station is itself marginal then the next strongest station will be retained. This two-threshold scheme has been found in practice to be pretty effective but has a drawback that it sometimes errs on the side of too much handoff. Too much handoff reduces capacity because of the excess number of forward traffic channels needed to support it. It also impacts the number of channel elements, such as CDMA modems, needed in the base stations.
Support for forward link power control differs between the cellular and PCS air interface standards. IS-95A and Rate Set 1 of J-STD-008 standards specify only messaging-based forward power control. That is, when the mobile radiotelephone concludes, because of excessive frame error rate, that its forward signal quality is poor, it sends a report to the base station. This method is relatively slow, being impacted by processing delay in the message parsing by the base station. Rate Set 2, the 14,400 bps set, incorporates a faster forward power control mechanism. Each reverse traffic frame incorporates a bit that reports erasures with a slight processing delay.
In some communication systems it is difficult to accomplish handoff from one base station to another without knowing the exact location of the radiotelephone. This is particularly true for CDMA systems where the handoff is to be done to the base station with a different carrier frequency. In a CDMA-based cellular system, only one carrier frequency is used for all radiotelephones in an area supervised by a base station, but the surrounding base stations may use a different carrier frequency. When the signal in a radiotelephone is weak, or becomes weak at the edge of the current cell, in a CDMA system in which each base station has a different carrier frequency, a new base station's transmittal of a new carrier frequency cannot be received by the radiotelephone with a weak signal, which does not know the new frequency. Since a handoff cannot be completed in that case, the radiotelephone becomes useless. Further, it may impair the ability of other cellular radiotelephones in the cell to transmit.
Therefore, it is necessary to determine which radiotelephone has a weak signal and to perform a handoff or a dropoff of that radiotelephone by the base station of the cell. With conventional methods, the base station can only determine that a weak radiotelephone is within a certain range, without knowing its exact location. Moreover, because the base station can only identify the radiotelephone in response to paging from the base station, it takes time to reach the radiotelephone with a weak signal.
FIG. 1 illustrates an exemplary cellular communication system cell network. It should be understood that in the actual cellular communication environment cells 10, 11 and 13 may vary in size and shape. The cellular system of FIG. 1 may be an analog or digital communication system, and may employ one or more of several types of multiple access modulation schemes, such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA).
For switching off a radiotelephone, it must be determined which radiotelephone 12 is powered on, and the cell base station 14 or 15 that the radiotelephone 12 is receiving from. To find the radiotelephone 12, the cellular system broadcasts a message, often called a page, in many cells 10, 11, 13. When the radiotelephone 12 responds, the cellular system continues handling the call with further communication being directed to one of the cells 10, 11, 13 containing the radiotelephone 12. If the system has no knowledge of the location of the radiotelephone 12, the system must broadcast pages in every sector of every cell 10, 11, 13. As the cellular communication system traffic increases, the communication resources to support system-wide paging in a large metropolitan area become huge, because each radiotelephone 12 has to register with at least one base station 14, 15 to inform the cellular system where it is located, and to become controlled and paged one or more base stations 14, 15.
When a radiotelephone 12 is turned on, it typically must search for and acquire a pilot signal, which is in a CDMA system continuously transmitted by the base stations 14. The pilot signal is used by the radiotelephone 12 to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the signals from the base stations 14. Pilot signal acquisition typically takes a few seconds. This is largely due to the proximity of the cell base stations 14 to the radiotelephone 12 and the fact that the cell 10 is stationary. When the strongest signal is found, the radiotelephone 12 registers with that cell's base station 14.
Referring to FIG. 2, when the radiotelephone 12 registers, it typically transmits, via its transceiver, identification parameters. These parameters may include its serial number and associated telephone number or numbers. The registration information is received at a transceiver located at a base station 14 for the cell 10 in which the radiotelephone 12 is located. The registration information is relayed on to a mobile telephone switching office (MTSO) 16 by the receiving base station 14. One function of the MTSO 16 is to route calls between the Public Switched Telephone Network (PSTN) 18 and various cells 10. MTSO 16 typically also contains a database with information relative to the cell structure.
In CDMA cellular communication systems, radiotelephones are paged in the various cells in order to identify the cell in which a radiotelephone is located, for call routing purposes. For this type of communication system, the system needs to determine the set of cells that the radiotelephone may have entered. A process known as registration enables a group of cells within which the radiotelephone would most likely be located to be identified. In the zone registration technique the amount of paging in the cellular system is reduced by dividing the system into zones and the radiotelephone is paged at all cells within the zone. For that purpose, the radiotelephone typically maintains a list of zones that it has recently visited, and, if it enters a zone not on the list, it then registers in a new zone. Every cell base station 14, 15, 17 broadcasts in the zone to which it is assigned. Upon radiotelephone registration in a particular cell, the MTSO uses the cell structure database to determine a zone, according to the distance of the radiotelephone from the cells. When a call intended for transfer to the radiotelephone is received at the MTSO, MTSO instructs the cells corresponding to the zone in which the radiotelephone is registered to transmit a paging message to the radiotelephone.
When paging by the base stations in the cells within a determined paging zone is accomplished, the radiotelephone, if still within the zone and in condition to receive the page, responds to the received page by transmitting a response message that is received by the base stations for the cells in which the radiotelephone is located or was previously located. The response to the page is relayed by the receiving cell base station to the mobile telephone switching office. The mobile telephone switching office identifies, through relay of the response by the cell base stations, the cell in which the radiotelephone is currently located. The mobile telephone switching office routes a call intended for the radiotelephone to the base station corresponding to the cell in which the radiotelephone is located.
In accordance with the conventional "distance based registration method", each cell base station transmits its location information and a predetermined limit distance value. In this method, each radiotelephone calculates a distance between the cell base station for the cell in which it is located, based upon the transmitted current cell base station location information, and a previous cell base station location information in which the radiotelephone was previously registered. The radiotelephone then checks whether the distance of the radiotelephone is greater than some predefined limit distance value from the previous cell, by comparing the computed distance with the limit distance value from the previous cell base station, and registers with the current cell base station when the computed distance is greater than the previous cell base station limit distance value. The registration information is relayed on to the mobile telephone switching office, where a determination is made of a zone of cells for paging of the radiotelephone. The radiotelephone stores in its memory the zone information.
Several systems, such as Advanced Mobile Phone System (AMPS) and GSM, use a timer or a counter method to determine the location of a radiotelephone. In a cellular system implementing the timer method, each radiotelephone registers with the base station every several seconds. The velocity of the movement of the cellular radiotelephone is estimated, and the approximate distance that it could have traveled is calculated from the time when the radiotelephone last registered. However, AMPS handoffs frequently fail, causing dropped calls, which contributes to perceptions of poor service quality. Moreover, each handoff is preceded and followed by long intervals of poor link quality, resulting in annoying noise and distortion.
In an alternative conventional method, base stations determine boundaries of their coverage areas with scaled contour shapes. The contour shapes have minimum and maximum boundaries which are generated based upon RF measurements of each base station. The intersections of the scaled contour shapes define a bounding polygon area that describes the spatial position of a cellular radiotelephone in terms of error estimates. The center of the polygon is determined, whereupon the radiotelephone location address may be determined through reference to a database. Yet another distance control CDMA method is used in satellite communication systems to provide a handoff by mobile radiotelephone's receiver, which drops one satellite and synchronizes to another, by employing the receiver's and transmitter's known positions to make a switch.
Presently, when a mobile radiotelephone is working in a digital CDMA mode and moving towards an AMPS-only area, which means that the base station only handles analog signals, a handoff has to be performed by switching the mobile radiotelephone from digital into analog mode, if the mobile radiotelephone has the capability, and the radiotelephone has to stay in analog mode which is not always preferable. Moreover, the IS-95A standard prescribes that all CDMA base stations and radiotelephones in the future will have to be equipped with both analog and digital mode. In such a system, when the mobile radiotelephone is in a disadvantaged reception area, where the digital transmission is weak and may drag down the other mobiles of the power-controlled CDMA system, a soft handoff will have to be performed by switching into an analog mode. However, the IS-95A standard does not provide the option of switching back into digital mode.